The present invention relates generally to processes and systems for embossing sheets of flexible graphite material, and more particularly to processes for reducing warping of the graphite material during the embossing process. The articles formed by the embossing process may be used to form components of an electrochemical fuel cell.
An ion exchange membrane fuel cell, more specifically a proton exchange membrane (PEM) fuel cell, produces electricity through the chemical reaction of hydrogen and oxygen in the air. Within the fuel cell, electrodes denoted as anode and cathode surround a polymer electrolyte to form what is generally referred to as a membrane electrode assembly, or MEA. Oftentimes, the electrodes also function as the gas diffusion layer (or GDL) of the fuel cell. A catalyst material stimulates hydrogen molecules to split into hydrogen atoms and then, at the membrane, the atoms each split into a proton and an electron. The electrons are utilized as electrical energy. The protons migrate through the electrolyte and combine with oxygen and electrons to form water.
A PEM fuel cell is advantageously formed of a membrane electrode assembly sandwiched between two graphite flow field plates. Conventionally, the membrane electrode assembly consists of random-oriented carbon fiber paper electrodes (anode and cathode) with a thin layer of a catalyst material, particularly platinum or a platinum group metal coated on isotropic carbon particles, such as lamp black, bonded to either side of a proton exchange membrane disposed between the electrodes. In operation, hydrogen flows through channels in one of the flow field plates to the anode, where the catalyst promotes its separation into hydrogen atoms and thereafter into protons that pass through the membrane and electrons that flow through an external load. Air flows through the channels in the other flow field plate to the cathode, where the oxygen in the air is separated into oxygen atoms, which joins with the protons through the proton exchange membrane and the electrons through the circuit, and combine to form water. Since the membrane is an insulator, the electrons travel through an external circuit in which the electricity is utilized, and join with protons at the cathode. An air stream on the cathode side is one mechanism by which the water formed by combination of the hydrogen and oxygen is removed. Combinations of such fuel cells are used in a fuel cell stack to provide the desired voltage.
One limiting factor to the use of graphite materials, especially flexible graphite materials, as components for PEM fuel cells is the definition of a pattern embossed on the material, which, if not sufficient, can interfere with operation of the fuel cell, by permitting leaking of fluids, or not permitting sufficient fluid flow through the fuel cell, or changing load and/or current paths through the cell.
Graphite""s are made up of layer planes of hexagonal arrays or networks of carbon atoms. These layer planes of hexagonally arranged carbon atoms are substantially flat and are oriented or ordered so as to be substantially parallel and equidistant to one another. The substantially flat, parallel equidistant sheets or layers of carbon atoms, usually referred to as graphene layers or basal planes, are linked or bonded together and groups thereof are arranged in crystallites. Highly ordered graphites consist of crystallites of considerable size: the crystallites being highly aligned or oriented with respect to each other and having well ordered carbon layers. In other words, highly ordered graphites have a high degree of preferred crystallite orientation. It should be noted that graphites by definition possess anisotropic structures and thus exhibit or possess many properties that are highly directional e.g. thermal and electrical conductivity and fluid diffusion.
Briefly, graphites may be characterized as laminated structures of carbon, that is, structures consisting of superposed layers or laminae of carbon atoms joined together by weak van der Waals forces. In considering the graphite structure, two axes or directions are usually noted, to wit, the xe2x80x9ccxe2x80x9d axis or direction and the xe2x80x9caxe2x80x9d axes or directions. For simplicity, the xe2x80x9ccxe2x80x9d axis or direction may be considered as the direction perpendicular to the carbon layers. The xe2x80x9caxe2x80x9d axes or directions may be considered as the directions parallel to the carbon layers or the directions perpendicular to the xe2x80x9ccxe2x80x9d direction. The graphites suitable for manufacturing flexible graphite sheets possess a very high degree of orientation.
As noted above, the bonding forces holding the parallel layers of carbon atoms together are only weak van der Waals forces. Natural graphites can be chemically treated so that the spacing between the superposed carbon layers or laminae can be appreciably opened up so as to provide a marked expansion in the direction perpendicular to the layers, that is, in the xe2x80x9ccxe2x80x9d direction, and thus form an expanded or intumesced graphite structure in which the laminar character of the carbon layers is substantially retained.
Graphite flake which has been chemically or thermally expanded and more particularly expanded so as to have a final thickness or xe2x80x9ccxe2x80x9d direction dimension which is as much as about 80 or more times the original xe2x80x9ccxe2x80x9d direction dimension can be formed without the use of a binder into cohesive or integrated sheets of expanded graphite, e.g. webs, papers, strips, tapes, or the like (typically referred to as xe2x80x9cflexible graphitexe2x80x9d). The formation of graphite particles which have been expanded to have a final thickness or xe2x80x9ccxe2x80x9d dimension which is as much as about 80 times or more the original xe2x80x9ccxe2x80x9d direction dimension into integrated flexible sheets by compression, without the use of any binding material, is believed to be possible due to the mechanical interlocking, or cohesion, which is achieved between the voluminously expanded graphite particles.
In addition to flexibility, the sheet material, as noted above, has also been found to possess a high degree of anisotropy with respect to thermal and electrical conductivity and fluid diffusion, comparable to the natural graphite starting material due to orientation of the expanded graphite particles substantially parallel to the opposed faces of the sheet resulting from very high compression, e.g. roll pressing. Sheet material thus produced has excellent flexibility, good strength and a very high degree of orientation.
Briefly, the process of producing flexible, binderless anisotropic graphite sheet material, e.g. web, paper, strip, tape, foil, mat, or the like, comprises compressing or compacting under a predetermined load and in the absence of a binder, expanded graphite particles which have a xe2x80x9ccxe2x80x9d direction dimension which is as much as about 80 or more times that of the original particles so as to form a substantially flat, flexible, integrated graphite sheet. The expanded graphite particles that generally are worm-like or vermiform in appearance, once compressed, will maintain the compression set and alignment with the opposed major surfaces of the sheet. The density and thickness of the sheet material can be varied by controlling the degree of compression. The density of the sheet material is typically within the range of from about 0.04 g/cc to about 1.4 g/cc. The flexible graphite sheet material exhibits an appreciable degree of anisotropy due to the alignment of graphite particles parallel to the major opposed, parallel surfaces of the sheet, with the degree of anisotropy increasing upon roll pressing of the sheet material to increased density. In roll pressed anisotropic sheet material, the thickness, i.e. the direction perpendicular to the opposed, parallel sheet surfaces comprises the xe2x80x9ccxe2x80x9d direction and the directions ranging along the length and width, i.e. along or parallel to the opposed, major surfaces comprises the xe2x80x9caxe2x80x9d directions and the thermal, electrical and fluid diffusion properties of the sheet are very different, by orders of magnitude typically, for the xe2x80x9ccxe2x80x9d and xe2x80x9caxe2x80x9d directions.
This considerable difference in properties, i.e. anisotropy, which is directionally dependent, can be disadvantageous in some applications. For example, in gasket applications where flexible graphite sheet is used as the gasket material and in use is held tightly between metal surfaces, the diffusion of fluid, e.g. gases or liquids, occurs more readily parallel to and between the major surfaces of the flexible graphite sheet. It would, in most instances, provide for greater gasket performance, if the resistance to fluid flow parallel to the major surfaces of the graphite sheet (xe2x80x9caxe2x80x9d direction) were increased, even at the expense of reduced resistance to fluid diffusion flow transverse to the major faces of the graphite sheet (xe2x80x9ccxe2x80x9d direction). With respect to electrical properties, the resistivity of anisotropic flexible graphite sheet is high in the direction transverse to the major surfaces (xe2x80x9ccxe2x80x9d direction) of the flexible graphite sheet, and substantially less in the direction parallel to the major faces of the flexible graphite sheet (xe2x80x9caxe2x80x9d direction). In applications such as electrodes for fuel cells, it would be of advantage if the electrical resistance transverse to the major surfaces of the flexible graphite sheet (xe2x80x9ccxe2x80x9d direction) were decreased, even at the expense of an increase in electrical resistivity in the direction parallel to the major faces of the flexible graphite sheet (xe2x80x9caxe2x80x9d direction).
With respect to thermal properties, the thermal conductivity of a flexible graphite sheet in a direction parallel to the major surfaces of the flexible graphite sheet is relatively high, while it is relatively low in the xe2x80x9ccxe2x80x9d direction transverse to the major surfaces.
Flexible graphite sheet can also be provided with channels, which are preferably smooth-sided, and which pass between the parallel, opposed surfaces of the flexible graphite sheet and are separated by walls of compressed expanded graphite. When such a flexible graphite sheet functions as an electrode in an electrochemical fuel cell, it is placed so as to abut the ion exchange membrane, so that the xe2x80x9ctopsxe2x80x9d of the walls of the flexible graphite sheet abut the ion exchange membrane.
One problem which is often encountered in the embossing of articles from sheets of flexible graphite materials, is that the articles can warp or tear due to the forces exerted upon the relatively thin sheet of flexible graphite material by the embossing rollers.
There is a continuing need for improved processes for the manufacture of graphite articles by various forming techniques such as rotary embossing, and there is a need for the prevention of the warping and tearing of the graphite materials during the embossing process. The present invention provides such improved processes, which are particularly useful in the manufacture of materials which can be formed into components of electrochemical fuel cells.
The present invention addresses these problems by providing raised stabilization surfaces surrounding the periphery of an embossing pattern on the embossing roller, so that the raised stabilization surfaces will help control movement of the flexible graphite sheet through the embossing rollers, and will also control the tendency of the flexible graphite material to move laterally away from the embossing pattern due to the pressures exerted upon the material by the embossing roller.
The present invention provides a process of embossing a sheet of flexible graphite material which may be resin impregnated. The process comprises:
(a) providing an embossing roller having an embossing pattern defined thereon with a raised stabilization area adjacent a periphery of the embossing pattern; and
(b) applying a stabilizing force to the sheet adjacent the periphery of the embossing pattern with the raised stabilization area and thereby limiting egress of the material laterally away from the embossing pattern.
The raised stabilization area preferably has a roughened surface to increase its grip upon the sheet of flexible graphite material. Thus, the raised stabilization area will grip the sheet and pull the sheet past the embossing area.
The raised stabilization areas may include circumferential portions which extend circumferentially around at least a part of the embossing roller, and they also include transverse portions which extend across a portion of the width of the embossing roller.
In another embodiment, a method is provided of embossing a sheet of flexible graphite material, which method includes:
(a) providing a pair of embossing rollers, including a primary embossing roller and a backup roller, the primary embossing roller having an embossing pattern defined thereon;
(b) providing at least one of said rollers with a first roughened surface area; and
(c) rotating the rollers, and gripping the sheet between the rollers with the roughened surface area and thereby pulling the sheet through the rotating rollers.
The roughened surface area is preferably a raised area. The raised area may have a variable height complementary to a height of an opposing surface of the other of the rollers, so that a substantially constant gap is provided between the raised surface area and the opposing surface as the rollers rotate.
Accordingly, it is an object of the present invention to provide improved processes for the manufacture of articles from sheets of flexible graphite material.
Another object of the present invention is the reduction of warpage of articles formed by roller embossing sheets of flexible graphite material.
And another object of the present invention is the provision of improved systems for pulling sheets of flexible graphite material between a pair of embossing rollers.
And another object of the present invention is the provision of improved fixed gap embossing rollers for use with sheets of flexible graphite materials.
And another object of the present invention is the provision of apparatus and methods for manufacturing of materials suitable for use as components of fuel cells.
Other and further objects, features and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the following disclosure when taken in conjunction with the accompanying drawings.